[ONNX] Documentation and typing annotations in registry
Updating the docstrings and type annotations as I walk through the code.
- Turned some comments into docstrings.
- Added type annotations for some functions in utils and the registry
- Removed direct function imports; importing functions makes name space collision easier to happen and refactoring/code analysis harder: https://google.github.io/styleguide/pyguide.html#22-imports
- Formatted touched files with black
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76255
Approved by: https://github.com/BowenBao
